Fatty acyl isethionates (e.g., cocoyl isethionates) are anionic surfactants highly desirable in personal care skin or hair cleansing products, particularly in personal care products, because they lather well, are mild to the skin and have good emollient properties. Typically, fatty acid isethionates are produced by esterification of fatty acids or by reaction of fatty acid chloride having carbon chain length of C8 to C20 with isethionate. A typical surfactant product containing fatty acyl isethionate contains about 45 to 95 wt. % fatty acyl isethionates and 0 to 40 wt. % free fatty acid, in addition to isethionates salts, typically at less than 5%, and trace (less than 2 wt. %) of other impurities.
A persistent problem with the ready use of fatty acyl isethionates in liquid compositions, however, has been the low solubility of these compounds in water. This is especially true for fatty acyl isethionate surfactant products containing high level of free fatty acid (10% by wt. or higher) and/or long chain fatty acyl isethionates (e.g., C14 and higher). The fatty acyl isethionate component tends to form insoluble surfactant crystals with the amount of crystals depending strongly on the storage temperature due to the wide range of dissolution temperatures of these crystals. This in turn may result in unstable liquid cleansers which exhibit very thick or very thin consistency at low and elevated temperatures.
It would therefore be of tremendous advantage to have compositions having consistent viscosity; as well as a way of manipulating compositional ingredients to ensure such consistent viscosity is obtained and that fatty acyl isethionate product, no matter what their free fatty acid content or their chain lengths, can be readily used. The present invention provides precisely such compositions and processes for making such compositions.
Specifically, the invention recognizes that the problem of inconsistent viscosity can be resolved by converting part or all of the fatty acyl isethionate surfactant crystals to surfactant liquid crystals wherein the liquid crystals occupy sufficient phase volume to ensure stability, said stability being defined by the absence of visible physical separation after two weeks of storage at 40° C. This is accomplished by using a specific combination of liquid crystal modifiers (e.g., fatty acids, fatty alcohols); and sufficiently high levels of a surfactant liquid crystal inducer (e.g., alkanolamide, alkylamineoxide) as a percent of total fatty acyl isethionate plus synthetic surfactant. The specific combination of liquid crystal inducer and liquid crystal modifier creates a consistent viscosity which allows fatty acyl isethionates product, regardless of free fatty acid content or chain length of isethionates, to be storage stable.
Acyl isethionate liquids do exist in the art. U.S. Pat. No. 5,415,810 to Lee et al., for example, discloses compositions comprising fatty acyl isethionates and zwitterionic surfactant (e.g., cocoamidopropyl betaines), presumably to help solubilize the isethionate and make an isotropic liquid. The reference separately teaches away from use of both alkanolamides (column 1, lines 27-30), and use of free fatty acids, especially longer chain fatty acids (column 2, lines 34-39), let alone the use of both specifically in combination.
U.S. Pat. No. 5,739,365 to Brody et al. and U.S. Publication U.S. 2004/0224863 both disclose use of ammonium counterion to help solubilize fatty acid isethionate.
U.S. Pat. No. 5,132,037 to Greene et al. (and related U.S. Pat. No. 5,234,619 and U.S. Pat. No. 5,290,471) disclose compositions with C8 to C22 acyl isethionates, synthetics, and free fatty acid, preferably C16 or higher. The liquid crystal inducers (e.g., alkanolamide) of the subject invention are not disclosed, nor is a process to use both liquid crystal modifiers and liquid crystal inducers specifically together to provide long term stability of acyl isethionates.
U.S. Pat. Nos. 5,952,286 and 6,077,816, both to Puvvada, disclose liquid cleansing compositions which may contain acyl isethionates and which comprise soluble, lamellar phase inducing structurant (e.g., branched fatty acid). While amides may be optionally used in a long recitation of optional ingredients, there is no teaching or disclosure that they need be used; that they must be used in combination with liquid crystal modifier to consistently enhance stability of acyl isethionate; let alone that they must be used in certain minimal amounts. It is noted that the lamellar inducing structurants claimed in the Puvvada patents are branched fatty acids or branched fatty alcohols such as isostearic acid or isostearic alcohol. According to the subject invention straight chain fatty acids and/or straight chain fatty alcohols must form the predominant amount of fatty acid and/or alcohol (e.g., modifier) which are used in combination with alkanolamide. In fact, branched liquid fatty acid and/or branched fatty alcohol are not required at all in the subject invention. If used, however, the amount should be limited to no more than 30 wt %, preferably no more than 20 wt %, even more preferably 10% or less of total fatty acids and/or fatty alcohols in the liquid composition of the invention.
None of the references, alone or together, teach or suggest compositions comprising acyl isethionate where acyl isethionate/fatty acid crystals are converted in part, or completely, to a surfactant liquid crystal using specific combinations of high levels of liquid crystal inducer (e.g., alkanolamide) and liquid crystal modifier (e.g., predominantly straight chain fatty acid) in order to provide acyl isethionate containing liquids, regardless of the fatty acid content or fatty acid chain length of the acyl isethionates surfactant; which compositions have a product viscosity less sensitive to temperature, and which compositions are stable at both low and elevated temperature storage conditions.